Liquid-crystal modulators are well known. They are most prevalently used in displays ranging in size from wrist watches to flat-panel displays on lap top computers. In such displays, the bias applied to the pixel of the multi-element cell, when used in combination with polarizers, determines whether the pixel absorbs or passes light. Since the output is directly viewed, the ratio of the light passed in the transmissive mode to the light passed in the absorptive mode need not be very high for the contrast between the two states to be readily discernible. This ratio of intensities or similar characteristics is referred to as the extinction ratio for a liquid-crystal cell.
Specialized liquid-crystal optical modulators are also known in which a single, well defined beam strikes the modulator and its intensity is modulated according to the electrical bias applied across the liquid-crystal cell. Many applications of optical modulators require a high extinction ratio.
A relatively new application of liquid crystals involves optical switches in a multi-wavelength optical communication, as is disclosed by Patel et al. in U.S. Pat. No. 5,414,540. In U.S. patent application Ser. No. 08/780,925 entitled Wedge-Shaped Liquid-Crystal Cell, filed Jan. 9, 1997 in the name of Jayantilal Patel, incorporated herein by reference in its entirety. Patel briefly introduces this type of liquid-crystal switch and further explains the necessity for precisely defining the gap in the liquid-crystal cell used in such a device. The liquid crystal is filled into the gap, and electrodes on opposing sides of the gap are selectively biased to control the optical polarization converting characteristics of the cell.
It is well known that the transmissivity I of light through parallel polarizers sandwiching a 90.degree. twisted nematic liquid crystal follows a dependence ##EQU1## where ##EQU2## where d is the effective thickness of liquid-crystal, .DELTA.n is the birefringence, that is, the difference between the extraordinary and ordinary refractive indices n.sub.e, n.sub.o, and .lambda. is the free-space wavelength of the light with the previously defined quantities. The nematic liquid crystal is twisted by 90.degree. when no electrical bias is applied across the cell. The transmissivity when a strong electrical bias is applied across the cell is equal to unity when the parallel-polarizer transmissivity I is defined as in Equation (1). The ratio of the biased transmissivity to the parallel-polarizer transmissivity I is often called the extinction coefficient although there is some ambiguity in the usage of the latter term. For the high value of extinction coefficient required for high-performance modulator, the parallel-polarizer transmissivity I needs to be minimized, and it depends upon the thickness d with the dependence defined in Equation (1). Although the transmissivity I is relatively small for values of u greater than 1, it assumes a zero (minimum) value only for a discrete set of parameters dependent upon the positive even integers ##EQU3## which can be alternately expressed as EQU u=1.732, 3.873, 5.916, (4)
The values stated in either Equation (3) or (4) are known as the first, second, and third minimum conditions respectively and represent conditions for which exact polarization conversion occurs.
Thus, only for discrete values of cell thickness d does the extinction coefficient assume theoretically infinite value.
Patel discloses that the gap size can be precisely controlled in a wedge-shaped liquid-crystal cell 10 illustrated in partial cross section in FIG. 1. The cell 10 includes two assemblies of respective glass substrates 12, 14 coated with respective electrodes 16, 18 and alignment layers 20, 22, as is common for most liquid-crystal devices. However, when the two assemblies are assembled together with a gap 24 therebetween, two different sizes of spacers are used so that gap 24 assumes the shape of a wedge, that is, of varying gap from the top to the bottom. A typical variation in the size of the gap 24 is between 6 and 8 .mu.m for the infrared optical switches contemplated by Patel. The angle of the wedge is exaggerated in FIG. 1. A nematic liquid crystal 26 is filled into the so defined wedge-shaped gap 24.
This wedge-shaped liquid-crystal cell 10 is usable with an optical beam 28 having a vertical dimension small compared to the size of the cell 10 along the wedge direction. The cell 10 is supported on a vertically movable support 30, and an adjustment means 32 vertically moves the support 30 and hence the wedge-shaped liquid-crystal cell 10 up or down until the beam 28 strikes the cell 10 at a position having an optimally sized gap. The optimal size can be determined by several optical means, as is explained in the parent application.
The wedge-shaped liquid-crystal cell allows the operational gap to be established to accuracies virtually unobtainable in planar cells because manufacturing introduces variations in thickness much larger than the required accuracy in gap thickness.
It has long been known that the performance of liquid-crystal devices is affected by temperature. Patel has described one electronic compensation scheme in U.S. Pat. No. 5,113,275 for compensating a liquid-crystal filter by adjusting the biasing voltage. Others have been described in U.S. Pat. No. 3,921,162 to Fukai et al., U.S. Pat. No. 4,128,311 to Smith et al., U.S. Pat. No. 4,460,247 to Hilsum et al., U.S. Pat. No. 4,625,163 to Germer, and U.S. Pat. No. 4,834,504 to Garner. All these schemes have disadvantages and do not take advantage of the unique geometry in a wedge-shaped liquid-crystal cell.
Temperature variations in a liquid-crystal cell can arise from a number of sources. However, it believed that thermal expansion of mechanical parts, which would affect the gap size d produces a much smaller effect than do thermal effects upon the liquid crystal itself. It is known that the refractive index of nematic liquid crystals generally follows the dependence shown in FIG. 2. Above an isotropic transition temperature T.sub.i, also known as the clearing temperature, the refractive index is isotropic and the useful nematic qualities are absent. Below the clearing temperature T.sub.i, the refractive index is represented by an upper curve 40 for extraordinarily polarized light, that is, n.sub.e, and by a lower curve 42 for ordinarily polarized light, that is, n.sub.o. The difference between the two refractive indices 40, 42 is the refractive index difference or birefringence .DELTA.n appearing in Equation (2). As was stated before, the value of the parameter u, which depends upon .DELTA.n, needs to be precisely controlled for zero transmissivity I. The temperature dependence of FIG. 2 shows that the refractive index difference is dependent upon the operating temperature. That is, the birefringence should be represented as .DELTA.n(T'). The two curves 40, 42 approach each other with increasing slope as the temperature approaches the clearing temperature T.sub.i with the result that the temperature dependence of the birefringence .DELTA.n becomes very high just below the clearing temperature T.sub.i. On the other hand, the temperature dependence becomes increasingly smaller further below the clearing temperature T.sub.i.
These effects suggest that the temperature effects can be minimized by choosing a liquid crystal having a clearing temperature T.sub.i far above the operating temperature T'. For high-speed liquid-crystal cells, however, this relationship introduces the disadvantage that the viscosity will be correspondingly increased at temperatures far below the clearing temperature. Nonetheless, for the filters and switches contemplated by Patel, switching speed is not a major consideration for the expected millisecond switching times so a high clearing temperature T.sub.i is preferred.
Nonetheless, even a reduced temperature dependence is considered excessive for the very high extinction ratios required in multi-wavelength switches. It is greatly desired to remove all temperature dependence in the transmissivity I.
Active temperature control of the liquid-crystal device would eliminate the temperature dependence exhibited in FIG. 2. However, it is estimated that the cell would need to be regulated to temperature variations of less .+-.0.5.degree. C. Although such close regulation is possible, the equipment it requires is expensive, bulky, and not appropriate for a fielded operation, and it further introduces problems such as condensation when the ambient temperature and moisture are significantly varying.